Fix global constant discovery after class name tokens

[LubuSeb]

When a global constant follows stdClass::class inside a function, the directory symbol scan can mistake the class name token for a declaration and lose the constant. The same can happen with identifiers beginning with class. This change checks the keyword boundary before entering type scope in both the PHP cleaner and the Turbo scanner.

The regression covers ::class, className(), and a real anonymous class. The native differential corpora also include the trigger. Thanks to @Victor-emil for identifying the cause and suggesting the boundary check in phpstan/phpstan#15482.

Validation: the focused PHPUnit test passes (3 tests); the source-locator suite passes (78 tests, 159 assertions, 1 skip); and the issue's two-file example analyzes without errors using this branch. PHP syntax and git diff --check pass. I could not run the native differential scripts locally because the Turbo extension is not installed.

Fixes phpstan/phpstan#15482

[LubuSeb]

CI note: the result-cache-parse-error E2E check is failing on this PR, but the same check failed on the untouched 2.3.x base commit before this PR. Its final assertion expects a circular-reference error and gets exit 0. I have not changed that E2E case or the relevant result-cache code.

The PHP 7.4 call syntax and Turbo version-pin failures from my first push have been corrected in the current head.